State of the Art
The elevated concentration of lead in drinking water has become a major public health concern. A recent sampling by the U.S. Environment Protection Agency (EPA) of 660 large public water systems found that about 32 million Americans in 130 cities drink water from systems that exceed the federal action limit of 15 parts per billion and that 10 cities exceed the limit by as much as 5 times. The natural waters in many small towns and rural areas in the United States, especially in the southwest, often contain a high concentration of lead and copper.
The long-term intake of excessive lead in water, along with exposure from lead-based paint and contaminated soil and dust, can build up in blood to result in a concentration of this toxic metal to a harmful level. Lead is known to severely hamper physical and mental development in children, to raise blood pressure and interfere with hearing and, at a very high level, to cause kidney damage and mental retardation in adults. In reaction to these findings, the EPA's rule concerning lead in drinking water has become much more stringent, requiring some 79,000 public water supply systems to monitor lead levels at the tap and setting an action level of 15 ppb. The lead may be present in the water as solid particles or be in the water as a soluble complex.
There are various methods to separate lead from the aqueous solution. The separation processes employed usually involve (1) ion exchange, (2) adsorption, (3) reverse osmosis, and (4) coagulation and precipitation.
Ion exchange is a process by which a given ion on an exchange solid is replaced by another ion in the solution, and is often used in processes for control of soluble metals, such as lead. For example, it is known that an ion exchange resin in calcium form can reduce lead in household drinking water, but the resin often lacks the ability to remove lead to the very low level required. Solid minerals can also be used as an ion exchange medium. For example, Takeuchi et al. ("A Study Equilibrium and Mass Transfer . . . ," Journal of Chemical Engineering of JAPAN, 21:1 pp. 98-100, 1988) discloses batch adsorption experiments using solid hydroxyapatite (Ca.sub.5 (PO.sub.4).sub.3 OH). Heavy metals, including lead, were removed from distilled water spiked with the metals by an equilibrium mass transfer of metals between the solid and aqueous phase. The removal of heavy metals was attributed to the ion exchange process in which the surface calcium was replaced by a divalent metal, e.g. Pb.sup.2+.
Adsorption processes usually exploit the Weak van der Waals forces which are responsible for many reversible adsorption of solutes to solid surface, or may involve more specific processes such as ion exchange and/or surface complexation. The weak physical adsorption can be easily reversed upon changes in conditions such as concentration of the solute, pH, temperature, or saturation of surface sites. Prior-art adsorption processes include using a granular activated carbon fixed-bed in a canister as a point-of-use device to remove lead from drinking water. Lead from a solution has also been adsorbed upon the surface of Vermiculite (a mica), Montmorillonite (a bentonite clay) and Goethite (an iron oxide).
Reverse osmosis has been used in point-of-use devices for removing lead from drinking water, as disclosed in Consumer Reports ("Water Treatment Devices," February 1993, pp. 79-82). Also disclosed are devices using distillation, and filtration.
Precipitation, where selected chemicals are applied to cause the solubility of solids to be exceeded, has been used to separate lead from the aqueous phase. Most particularly, carbonate or hydroxide precipitation has been proposed to remove heavy metals from solution. For example, calcium carbonate added to lead solutions has been used to remove lead as a precipitate. It has also been proposed to remove lead by coagulation and flocculation with alum at pH 8 to 9.
In U.S. Pat. No. 5,098,579 to Leigh et al. a method is proposed for continuously treating water by contacting the water with a metal salt which is sparingly soluble in water and has a very strong affinity to react with the ions to be removed to form an insoluble salt. The choice of the sparingly soluble salt is based upon the properties of the ion to be removed. For removal of Pb.sup.2+ ions the sparingly soluble salt may be any of various carbonate and chromate salts, Ca.sub.3 (PO.sub.4).sub.2, CaSO.sub.4, or mixtures thereof.
The precipitation processes, such as Leigh et al., are directed mainly to industrial waste streams, and the like. Typically, prior-art precipitation treatments of water were not designed to remove lead to an extremely low value, such as to 15 ppb, and were not designed to function in a point-of-use home culinary system. An ideal point-of-use device for removing lead from drinking water should be capable or removing lead to a concentration of 15 ppb or lower. In addition, it should be relatively inexpensive, mechanically simple and not involve much maintenance. A device that is awkward to apply to existing drinking water systems in the household, has a short service life or requires frequent recharging, involves handling of chemicals, particularly hazardous solids and liquids, or is expensive to purchase or maintain is not suitable. Such a device is will likely not be used at all and will be eventually discarded or misused by the consumer. In addition, a point-of-use system should provide some indication when its lead removing ability is exhausted. For example, in systems using adsorbents, the like, there is usually no indication when the adsorbent is approaching saturation and becoming ineffective in removing lead sufficiently from the drinking water.
For use in culinary water, it is necessary to use a system that not only removes the toxic lead ions, but also does not introduce substances that are themselves toxic or will give the water a bad taste. For example, it has been found that Ca.sub.3 (PO.sub.4).sub.2 can be used to reduce lead to below the EPA action level, but it leaves a concentration of phosphate ions that gives the water a strong taste. The phosphate ion concentration can be eliminated or reduced by using carbonate salts or by mixing phosphate salts with carbonate salts. However, it has been found that this increases the solubility of the lead ions to a concentration above the EPA action level. Thus, practice of a prior-art precipitation system, such as the Leigh et al. system, for removing lead ions either will not remove lead to sufficiently low levels, or it will produce water with safe lead levels but is bad tasting.